Ion exchange membranes are used as solid electrolytes in electrochemical energy conversion devices such as fuel cells, in electrolyzers, in separation processes, in sensors, and in many other applications. Perfluorinated ion exchange membranes are state of the art proton exchange membranes (PEM) used as electrolytes for fuel cells. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, robust, highly proton conductive, and gas impermeable. Such membranes are generally prepared by solution or dispersion casting of the acid form of the ionomer or by melt-extrusion of a precursor polymer containing sulfonyl halide protective groups that have to be hydrolyzed and acid-exchanged after melt-processing.
Extruded membranes have already proven to have a much higher mechanical and chemical durability in a fuel cell than solution-cast membranes (Lai 2009). The melt-processes are scalable to high volume production, representing significant reduction in manufacturing cost and provide robust self-reinforced membranes. Extrusion by melt-casting has been used to prepare proton exchange membranes, available commercially as Nafion™ N-117, Nafion™ N-115, Nafion™ N-1135, Nafion™ N-112, and Nafion™ 111-IP with thicknesses of 183, 127, 89, 51 and 25 microns, respectively. These extruded membranes also suffer from anisotropy in their properties in general, that may cause a premature failure when submitted to humidity cycling in a fuel cell. Extruded membranes are prototyped using a melt-casting process that generates strong orientation in the machine direction responsible for the anisotropy. Furthermore the extrusion process by melt-casting, does not allow the manufacturing of membranes thinner than 25 microns without compromising thickness uniformity.
There remains a need for durable, low-cost proton exchange membranes with reduced thickness that may reduce ohmic resistance in a fuel cell and improve the overall performance.